Osmoregulation controls how much water is lost in urine, and so controls that amount of water I the body.

Sensitivity

The human nervous system includes: the central nervous system- brain and spinal cord. The sense organs, such as the eyes and ears, which contain receptors that can detect a change in the environment and produce and electrical impulse. The nerves that join the central nervous system to the sense organs and effector organs- these are made up of bundles of nerve cells, or neurones.

There are three main types of neurones. Sensory neurones carry impilses to the central nervous system. Motor neurones carry impulses from the central nevous system tp effector organs. Relay neurones are found only in the central nervous system.

Responding to Stimuli

Some parts of the skin are more sensitive than others. This can be tested by touching different parts of the skin with two points and finding out how far apart the points have to be so that they can be detected as two points instead of just one.

A synapse is the point where two neurones meet. There is a small gap between the neurones. The electrical nerve impulse cannot cross this gap, and the impulse is carried by transmitters.

1) Electrical nerve impulse reaches end of axon. 2) Electrical impulse causes a chemical neurotransmitter to be released into the gap between neurones. 3) Neurotransmitter causes new electrical impulse in next neurone.

Hormones

Hourmones are chemical messengers. They are produced by endocrine glands and released into the blood. They travel around the body in the blood until they reach their target organs. The hormone then causes the target organ to respond, e.g. by releasing another chemical. Different hormones have different target organs and cause different responses.

Blood glucose regulation is another example of homeostasis. It is controlled by two hormones: insulin and glucagon

After a meal the glucose concentration, the pancreas detects rise in blood glucose concentration. Pancreas increases secretion of insulin and decreases secretion of glucagon. Insulin causes muscle and live cells to remove glucose from blood and store it as glycogen. Blood glucose level falls.

A person who cannot control their blood glucose concentration properly has a condition known as diabetes.

Type 1 diabetes: People with type one diabetes do not produce any insulin in their pancreas and so have to inject insulin into subcutaneous fat. They have to work out the right amount of insulin to inject so that blood glucose concentration is kept within safe limits.

Type 2 diabetes: People with type two diabetes make insulin, but their liver and muscle cells have become resistant to it. Most people with type two diabetes control their blood glucose concentration by eating less foods that contain sugar and exercising.

BMI stants for body mass index and is calculated using the equation: BMI= weight in kilograms/ height in metres squared. People who have a BMI over 30 are said to be obese. Obesity is linked with many health problems, including type 2 diabetes.

Plant Hormones

A tropism is a plant's response to a stimlus by growing. A positive tropism is when the plat grows towards the stimulus. Plant shoots show a positive phototropism as they grow towards the light. Plant roots show positive gravitropism as they grow downwards- towards the pull of gravity.

Plant hormones or plant growth substances are chemicals that cause changes in plants. Axins make cells grow longer. Gibberellins can make plant shoots grow longer. They also control when seeds germinate.

Auxins are affected by light and cause phototropism in shoots. In a shoot, where light is coming from one side: auxin in produced in cells ear the top of a shoot. Auxins move to shaded part of shoot. Auxins cause the cells to elongate.